This invention relates generally to digital electronic systems, in particular waveform restoration systems used therein. Digital electronic systems utilize information encoded into sequential binary signals existing in either of two digital logic "states" to communicate or translate information between a number of information processing system components. A typical digitally encoded signal comprises a plurality of "squared" pulse-like signals. In order to effectively utilize such digital signals, a great number of encoding languages have been developed each being composed of combinations or sequences of binary two-state signals. While the languages used and systems processing them vary greatly in content format and encoding systems, all share the common requirement of a "clear" transition to perform effectively.
However, even within the most rudimentary digital logic systems a number of noise or extraneous signal sources are constantly in action which often contaminate the digital signal. Unless the effect of such noise is eliminated erroneous system performance can result. One common source of such noise is found in the transients which accompany electrical switching. Most typically these transients arise as electrical contacts mate or part producing a "spike" that is a sharp, short duration signal. Transients also arise due to contact bounce which produces a series of closely spaced noise pulses. In either case the noise signals combine with the digital information and are often interpreted as desired signals by the system. This, of course, gives rise to erroneous system output.
Digital logic systems are complex and perform their interpretive and calculative processes as well as information storage and retrieval through the use of great numbers of information translating devices. They inherently require a great variety and number of switches and switching systems to properly implement system operation. Even within the most sophisticated and up-to-date digital logic systems switches are used to route signal and information which must of necessity make and break a great number of electrical connections which give rise to transient or spike responses.
In addition to switching transient problems noise may be produced outside the logic system. For example, one of the most promising yet challenging environments for digital systems is the utilization of the vast telephone network as a distribution system. While the potential for digital system use of the telephone distribution system is great, the challenge presented to a digital system in utilizing the telephone distribution network is also great. Telephone systems are in essence vast switching networks which during normal performance are extremely laden with switching transient noises making their use by digital information systems difficult.
These and other noise problems have led practitioners in the art to develop numerous signal conditioning systems, the simplest of which is a passive integrating network such as set forth in U.S. Pat. No. 3,466,647, issued to John Guzak, Jr. In the system shown a number of keyboard switches are used to couple information into digital logic circuitry. Guzak provides a plurality of capacitors in parallel with the switches which integrate the switching transients and dampen switch action. It is also known to utilize active integrators such as those shown in U.S. Pat. No. 3,405,286, issued to R. Mudie, and U.S. Pat. No. 3,792,363, issued to Gebel et al. Such active integrators comprise combinations of an inverting amplifier stage and A.C. feedback network (such as a capacitor) which couple a portion of the output signal back to the input terminal to produce negative feedback. The basic idea is to provide a negative feedback or gain reduction for signals of higher frequency typical of those forming transient or noise spikes within the signal. While both active and passive type integrators can under many conditions effectively reduce the high frequency transient noise content of the digital binary encoded signal they unfortunately also introduce an often undesirable increase in the rise and fall times of the digital signal.
U.S. Pat. No. 3,513,333, issued to R. T. Andersen, sets forth an inverting amplifier having a positive feedback system which is directed primarily to minimizing the effects of noise transients produced by "contact bounce" within the system. As mentioned, contact bounce is present when a mechanical circuit breaker separates or mates ineffectively and produces a series of connect and disconnect transients in short succession after switch actuation. Andersen recognizes this fact and uses an amplifier in combination with a positive A.C. feedback network to supply a hold-off voltage causing the amplifying network to ignore noise signals occurring during the interval following switch actuation. While the described system is somewhat effective against the particular family of noise produced by switch contact bounce, the inverting amplifier and positive feedback offers little in the way of protection from noise transients occurring at other times within the system.
Still another apparatus for reducing the noise within digital systems is set forth in U.S. Pat. No. 3,824,583, issued to Quentin C. Turtle, which discloses a monostable multivibrator circuit (often called a "one-shot" multivibrator) which is characterized by the generation of an output signal pulse in response to an input trigger signal. The output signal has a duration independent of the input trigger signal duration. One of the primary benefits of such one-shot multivibrators arises from their inherent rejection of any trigger during the duration of their output pulse; however, they will still trigger at other times on noise pulses. A somewhat similar device well known in the art and often used to remove noise from digital encoded information signals is that of a Schmitt trigger. While somewhat similar to a monostable multivibrator, Schmitt trigger circuits tolerate more noise due to their displaced thresholds and enhance rise times due to their positive feedback. Schmitt triggers are often referred to as "squaring" circuits. Again, their use in digital systems is limited due to their displaced thresholds and their inability to reject large noise pulses. Both monostable multivibrators and Schmitt triggers offer some advantages in the art of noise suppression. However, their circuit construction is generally complex and limited in performance and the need arises for a simpler, more effective waveform conditioning circuit within the digital art.
Accordingly, it is an object of the present invention to provide an improved waveform conditioning system for use in a digital electronic information system. It is a more particular object of the present invention to provide an improved waveform conditioning network which utilizes a minimum number of system components and produces effective noise elimination without substantial deleterious effect upon the rise and fall times of the digital signal.